Ablative material removal is especially useful for medical purposes, either in-vivo or on the outside surface (e.g., skin or tooth), as it is essentially non-thermal and generally painless. Moreover, ablative material removal essentially exerts no pressure on the surface of the material, so it is quite useful for many other types of cutting and machining. Ablative material removal is generally done with a short optical pulse that is stretched amplified and then compressed. A number of types of laser amplifiers have been used for the amplification, including fiber amplifiers. Fiber amplifiers have a storage lifetime of about 100 to 300 microseconds. While some measurements have been made at higher repetition rates, these measurements have shown an approximately linear decrease in pulse energy. For ablations purposes, fiber amplifiers have been operated with a time between pulses of equal to or greater than the storage lifetime, and thus are generally run a repetition rate of less than 3-10 kHz.
Laser ablation is very efficiently done with a beam of very short pulses (generally a pulse-duration of three picoseconds or less). While some laser machining melts portions of the work-piece, this type of material removal is ablative, disassociating the surface atoms. Techniques for generating these ultra-short pulses are described, e.g., in a book entitled “Femtosecond Laser Pulses” (C. Rulliere—editor), published 1998, Springer-Verlag Berlin Heidelberg New York. Generally large systems, such as Ti:Sapphire, are used for generating ultra-short pulses (USP). When high-power pulses are desired, they are often intentionally lengthened before amplification to avoid thermally-induced internal component optical damage. Techniques for surface gratings are described in “Zero Reflectivity High Spatial Frequency Rectangular Groove Dielectric Surface Relief Gratings” by Thomas Gaylord, et. al. Dec. 15, 1986, Applied Optics, Vol. 25, pp. 4562-4567.
USP phenomenon was first observed in the 1970's, when it was discovered that mode-locking a broad-spectrum laser could produce ultra-short pulses. The minimum pulse duration attainable is limited by the bandwidth of the gain medium, which is inversely proportional to this minimal or Fourier-transform-limited pulse duration. Mode-locked pulses are typically very short and will spread (i.e., undergo temporal dispersion) as they traverse any medium. Subsequent pulse-compression techniques are often used to obtain USP's. A traditional diffraction grating compressor is shown, e.g., in U.S. Pat. No. 5,822,097 by Tournois. Pulse dispersion can occur within the laser cavity so that compression techniques are sometimes added intra-cavity. Previous approaches have generally operated maximum-sized amplifiers at maximum power and amplified longer and longer pulses. When high-power pulses are desired, they are intentionally lengthened before amplification to avoid internal component optical damage. This is referred to as “Chirped Pulse Amplification” (CPA). The pulse is subsequently compressed to obtain a high peak power (pulse-energy amplification and pulse-duration compression).